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Hum from speakers

clownfish

It's all in the details
I have an Optonica SM-7100 integrated amplifier from 1981 that now seems to make the speakers hum gently and I would like to be able to stop it.

The gentle but noticeable hum persists even when all sources are disconnected. It is unaffected by the volume control and moving the speakers (and cables) around has zero effect. My guess is it's at 50 Hz so sourced from the mains.

No hum from the toroidal transformer in the amp or anywhere else and the amp works perfectly in all other respects.

Is it just an amp issue, maybe something getting fed to the output stages? Any suggestions for resolving this because I'm out of ideas.
 
@clownfish

Favourite amps are hard to find.

If you elect to have all of the EL caps replaced - entirely appropriate at the age of the amp - you will likely not only solve your hum problem, but also enjoy a very significant sonic upgrade.

A competent service provider will also give the rest of the circuit a check, clean dirty pots and check for aging or heat problems.

Many of the new components now available - EL capacitors included - will easily outperform the original parts when they were new; and having everything back to specification might be a very positive revelation.

This will probably require 24-hours to 48-hours of continuous amplifier operation, following surgery, to allow the new capacitors to form and make friends. (Be warned! It might sound pretty terrible prior to this initial burn-in…)

Favourite amps are hard to find. If this is indeed a favourite amp, don’t hesitate.
 
Thanks. That makes sense and is really helpful.

Any suggestions on who to trust with this delicate operation. I’m based in Hampshire but could package really well and post or even deliver if it’s within a couple of hours drive.

Any thoughts on likely costs?
 
@clownfish

Did you have any joy solving this hum problem.

I’m hoping that you managed to find a quality EE to refresh the electrolytics and give the amp a well-earned tune-up.

How did you go?
 
@clownfish

Did you have any joy solving this hum problem.

I’m hoping that you managed to find a quality EE to refresh the electrolytics and give the amp a well-earned tune-up.

How did you go?
I’ve done nothing so far. It’s all ok when playing music as the hum can’t be heard. it does drive me a bit nuts when I pause playing for a bit and I really should get it sorted. Budget is a bit of an issue right now but hopefully later this summer I’ll get it sorted out.
 
What are the grounding/earthing arrangements, and what 'class' is the amp in those terms? Did you short the selected input when you tried having no sources connected?
 
I remember that amplifier well. The last dealership I worked for just post uni sold Optonica and that model was a real 'sleeper'. Looking inside, one can see how much effort Sharp put into this. The only real drawback from a long-term servicing perspective are the integrated output transistor packs (seen attached to the heat sinks either side, rather than 2 or 4x discrete transistors) which are nowadays unobtanium outside of cannibalistic circles. Still, this amp certainly deserves a quad of fresh power supply reservoir caps.

Optonica-SM-7100.jpg
 
What are the grounding/earthing arrangements, and what 'class' is the amp in those terms? Did you short the selected input when you tried having no sources connected?
I did not. And interesting, thank you ...

Should I short left and right together, or both to earth?
 
I remember that amplifier well. The last dealership I worked for just post uni sold Optonica and that model was a real 'sleeper'. Looking inside, one can see how much effort Sharp put into this. The only real drawback from a long-term servicing perspective are the integrated output transistor packs (seen attached to the heat sinks either side, rather than 2 or 4x discrete transistors) which are nowadays unobtanium outside of cannibalistic circles. Still, this amp certainly deserves a quad of fresh power supply reservoir caps.

Optonica-SM-7100.jpg
Is there a picture of the underside? I'm wondering how easy those PSU caps are to access as I can use a soldering iron reasonably well.
 
That's a great steer - I assumed he only handled a short range of valve kit!

I also see s/h Optonica SM-7100s for sale cheaply - that could be Plan B.
Thanks for the suggestion.

I guess my attachment is because I bought it when I was 17 and it's been a great source of joy to me over the years. I could buy another SM 7100 if spares were needed, although realistically, if it gives up the ghost I might plug my Optonice RP 7100 into a Rega Brio, or maybe another small amp, via my unused Arcam phonostage.
 
I did not. And interesting, thank you ...

Should I short left and right together, or both to earth?

Short each to ground at the socket. If that makes a difference see if leaving one shorted and the other open give more hum/buzz on one channel.

Do you have a DVM?
 
FWIW I normally earth systems via the preamp mains lead earth. Then transfer that to other items in the system via the outer of the coax cables. But this depends on the 'class' of each unit and what their designer expected/assumed. And some people may tut-tut about grounding via the coax outers.

In your case the amp is integrated. So if you put shorting plugs into the selected inputs AND ensure no other source items are physically connected you should get no hum at the output.

Point there is that some other item you have connected may also be earthed via its mains plug+lead. That can cause a loop which generates hum. If so, you may have a clash between what may be needed for 'safety' and hum. Or it may simply be a 'poor contact' somewhere in that loop - e.g. tarnished connection.
 
Hi Clownfish,

If the hum is a steady low 50 Hz or 60 Hz hum, it sounds like an Earth loop hum. Turn everything off then unplug all other equipment from the wall socket (except the amplifier), then unplug all inputs from the amplifiers’ input sockets as well, including the Turntable signal earth lead at the amplifier end if you have one.

Turn on the amplifier with nothing else connected and see if you still get the hum.

If the amplifier goes silent. You have an earth loop issue.

Reconnect each component one at a time listening for hum as you go to identify the offending component.

Regards
LPSpinner
 
Only caveat to the above is as you plug stuff into the RCA sockets on the amp, make sure you de-select the input that you are plugging into. RCAs make signal contact before earth and you will invariably get a VERY LOUD buzz as you plug the leads in.
I know this because I had an intermittent earth problem with a lead which when 'open' gave me a right bl00dy scare!
 
Hi Clownfish,

If the hum is a steady low 50 Hz or 60 Hz hum, it sounds like an Earth loop hum. Turn everything off then unplug all other equipment from the wall socket (except the amplifier), then unplug all inputs from the amplifiers’ input sockets as well, including the Turntable signal earth lead at the amplifier end if you have one.

Turn on the amplifier with nothing else connected and see if you still get the hum.

If the amplifier goes silent. You have an earth loop issue.

Reconnect each component one at a time listening for hum as you go to identify the offending component.

Regards
LPSpinner

Thanks for the suggestion.

Did this and I still get hum. Also shorted the phono sockets on the selected input and still get hum.
Selected the Mute button too and no change. Still humming :(
 


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